1. Field of the Invention
The present invention relates to a piston for use in a gas pump, to be slidably inserted in a cylinder which has a head and an outlet valve located close to the head. In particular, the invention relates to an improvement of the butting edges of the wear-reducing sheet wound one time around the periphery of the piston.
2. Description of the Related Art
FIG. 1 shows a typical piston for use in a gas pump. This free piston is rotatably and slidably inserted in a cylinder. When the poston moves toward the upper dead point, or moves leftward in FIG. 1, the compressed gas pushes open the outlet valve located in the vicinity of the head of the cylinder, and is thus exhausted from the cylinder through the outlet valve. As the piston further approaches the upper dead point, it closes the outlet valve. Once the outlet valve has been thus closed, the residual gas is further compressed within the pressure chamber defined by the top of the piston and the inner surfaces of the cylinder head. When the piston reaches the upper dead point, the compressed gas functions as a cushion, preventing the piston from abutting against the inner surface of the cylinder head.
When such a free piston is incorporated in a gas pump or a compressor, a wear-reducing sheet, which is resistant to friction and heat, is wrapped around the periphery of the piston, thereby to reduce the wear of the piston. The wear-reducing sheet must be wound around the piston such that its curvature is exactly the same as that of the periphery of the piston and the inner periphery of the cylinder. Even a slightest difference in curvature between the sheet and the inner periphery of the cylinder cannot be overlooked. Hence, the sheet must be wound around the piston, with both edges abutting on each other, not overlapping each other.
During the operation of the pump or compressor, the free piston reciprocates within the cylinder it is possible that the wear-reducing sheet undergoes a volume thermal expansion due to the heat resulting from the friction between the sheet and the inner periphery of the cylinder. To prevent the sheet from deforming because of such a volume thermal expansion, it is necessary to wrap the sheet around the piston, with a narrow gap left between the edges of the wear-reducing sheet. This causes a problem however. When the piston rotates within the cylinder such that this gap comes into alignment with the port of the outlet valve, the compressed gas in the pressure chamber flows out, first though the gap and then through the outlet valve. Consequently, the cushioning effect of the compressed gas is inevitably reduced.
It is true that the wear-reducing sheet protects the free piston from wear and heat. Nonetheless, it can cause a leak of gas unless it is appropriately wound around the free piston. If the gas leaks through the gap between the edges of the sheet, it generates a buzzing sound as it passes through the outlet valve.